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Modern airliners generally have lower cruise speeds than those of comparable aircraft from 40 years ago (the venerable Boeing 737 seems to be something of an exception; it has always had a more leisurely cruise speed).

Airlines had to respond much faster to the oil crisis than manufacturers could - to what extent were airlines able to economise on fuel consumption by flying more slowly, and what sort of margin was available to them at cruise (and by extension, what sort of margin is available in modern aircraft, should economic factors drive operators to seek lower fuel consumption by flying more slowly)?

Daniele Procida
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2 Answers2

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As a French who grew up in the Caribbean, I can tell that flights are getting longer. A flight between France and my island that took 7.5 hours 15 years ago is almost 9 hours long today.

This post published by the MIT’s School of Engineering says (emphasis mine):

Specified cruising speeds for commercial airliners today range between about 480 and 510 knots, compared to 525 knots for the Boeing 707, a mainstay of 1960s jet travel. Why? “The main issue is fuel economy,” says Aeronautics and Astronautics professor Mark Drela. “Going faster eats more fuel per passenger-mile. This is especially true with the newer ‘high-bypass’ jet engines with their large-diameter front fans.”

Today's high bypass engines are more efficient at low speed:

Observant fliers can easily spot these engines, with air intakes nearly 10 feet across, especially on newer long-range two-engine jetliners. Older engines had intakes that were less than half as wide and moved less air at higher speeds; high-bypass engines achieve the same thrust with more air at lower speed by routing most of the air (up to 93 percent in the newest designs) around the engine’s turbine instead of through it. “Their efficiency peaks are at lower speeds, which causes airplane builders to favor a somewhat slower aircraft”

This article also states that slower aircraft's structures are cheaper to build:

“A slower airplane can also have less wing sweep, which makes it smaller, lighter and hence less expensive.” The 707’s wing sweep was 35 degrees, while the current 777’s is 31.6 degrees.

Quentin H
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    Interesting information, and thanks, but I don't think it actually addresses the question: did airlines start to fly their planes more slowly? (which is not the same as did airline manufacturers start to design slower planes?). – Daniele Procida May 28 '19 at 18:02
  • If modern engines are more efficient at lower speeds, I imagine older engines were so as well. Just about any engine is more efficient in a region below it's maximum capacity. – Mast May 28 '19 at 18:03
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    @DanieleProcida airlines bought slower aircrafts overtime :) – Quentin H May 28 '19 at 18:03
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    I find this a bit dubious, as airliner cruise 'speeds' are rarely specified in knots. My understanding is that it's usually rather a specified cruise Mach number, which represents a different airspeed depending on altitude and atmospheric conditions. – reirab May 28 '19 at 18:05
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    @Mast Older engines were low bypass which makes them more efficient at higher speed – Quentin H May 28 '19 at 18:06
  • @Mast The 707 used low-bypass turbofans (bypass ratio around 1.4:1,) not high-bypass turbofans. – reirab May 28 '19 at 18:10
  • Right. Besides that, any aircraft is likely to be most efficient in the range it's supposed to work in (cruising speed), so going above or under it is likely to be inefficient anyway. – Mast May 28 '19 at 18:11
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    @QuentinHayot Yes, they did buy slower aircraft eventually - but can you add to your answer to say what they did at the time with the aircraft that they already had? – Daniele Procida May 28 '19 at 18:18
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    The 7.5 to 9 hour bit at the beginning of the answer also surprises me. I'm not surprised that it's a bit slower (the 747 was the main option 15 years ago and it's one of the fastest airliners out there, so replacing it will slow things down a bit,) but by that much seems surprising. I would guess that at least part of it is explained by other factors, such as difference in time of year (and thus wind), more congestion at the airports, routing, and schedule padding. Of course East-bound vs. West-bound will also be significantly different due to prevailing winds. – reirab May 28 '19 at 18:27
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    @Mast The 707 would have been most efficient at its (higher) cruise speed, but would still have been much less efficient than newer airframes with high-bypass engines at their most efficient (lower) cruise speeds. – J... May 28 '19 at 18:28
  • @reirab I did the round trip at the same time of the year every year all my life. The 747 was definitely quicker. I remember it was consistently 7.5 hours in one way and 8 hours on the way back. Today we fly 777's or A330/A350 and its around 8.2 and 8.5 up to almost 9 sometimes. And they can definitely go quicker because it happens that they will drop to 8 if they are very delayed and need to make up for it. – Quentin H May 28 '19 at 18:35
  • @QuentinHayot Yes, the 747s are definitely faster, just not enough to go from 7.5 hours to 9 for that reason alone. The 747 (-400 and -8) cruises at Mach 0.855. The 777 is Mach 0.84 and the A330 Mach 0.82. And you are correct that this is substantially less than maximum Mach number in each of these cases, so they can fly a bit faster if they need to at the expense of a bit more fuel... assuming that traffic allows. – reirab May 28 '19 at 18:42
  • @reirab you are probably right and my memories may compare best/worst performance. But there is definitely at least 1 hour of gap regardless of the Mach value. Maybe they don't fly exactly the same routes or whatever. – Quentin H May 28 '19 at 18:46
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    15 years ago was also just when the EU air passenger rights regulation was introduced. As its implications become clearer (and especially after the ECJ ruled that the cash compensation the regulation sets out for cancellation should also be paid in case of delays), airlines would have a definite incentive to construct their long-haul schedules more conservatively. Nobody complains when their flight arrives early. – hmakholm left over Monica May 28 '19 at 19:11
  • Contingency, and not so much from the 70's but the influence of Jan Carlzon in the 90's. Getting the cost below 10c per pax per Km factors in, yes. But it is not free to use airports and schedules matter. Building in a contingency helps pilots make up time. I would like to think they burn less fuel carrying fuel as well, but they also need that fuel for contingency. – mckenzm May 28 '19 at 22:34
  • It would be interesting to see the math on fuel savings per flight vs fielding more flights with the same fleet. Obviously personnel, maintenance / inspection times, etc make things a lot more complicated than just fuel efficiency alone. – brichins May 29 '19 at 19:39
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I have only my own anecdotal evidence. I was flying domestically & internationally quite extensively during the 1970s, and particularly the period spanning the oil crisis.

Although the impact of the 747 on mass air travel had begun in some markets, most aircraft were still legacy assets, often belonging to government owned national carriers that were still run by pilots, engineers & bureaucrats, not accountants & MBAs. It was very common on long international flights to have aircraft that were only half-full, if that; my first flights between Australia & the US or Europe were spent stretched out over three seats, this was considered to be commonplace at the time.

The airlines reacted to the oil crisis by immediately cutting the number of flights, & getting rid of a lot of bulk-buy cheap ticket deals to travel agents. In many cases it was a physical shortage of fuel as well as the increased cost that excused what led to sudden huge increases in queues at airports that were still just glorified empty hangars that had never dealt with crowds.

Long distance aircraft were constrained much more by having to stop & refuel much more often than they do now; what is today a single hop from Sydney to DFW, was then Sydney to Fiji, then Hawaii, then LA, then domestic.

I don't recall the aircraft going much slower, or at a different altitude (not sure if that was practical), but there may have been some adjustments with somewhat less flexible legacy aircraft such as the 707s & DC8s. They were already flying as efficiently as they could, just to avoid having to land. With fewer aircraft flying, they were now more likely to be operating closer to their maximum weight, which constrained performance.

Early 747s were more economical to start with, but when they were first introduced they had far fewer seats than post-oil crisis, when they were quickly reconfigured to get very crowded. Pre-crisis you preferred a new 747 for its space; post-crisis you hoped for a DC8 because they couldn't rob you of seat width.

With domestic air travel, again, they just cut flights & charged like wounded bulls. There were many non-flying areas in their operations that were hugely inefficient that could be made less costly, & these were all overhauled. Political lobbying became much more important in order to reduce technical & employment regulatory "costs". Refuelling at lower-cost airports & tankering changed scheduling.

TLDR, yes, they probably slowed aircraft slightly, but that wasn't nearly enough - instead the airlines cut flights massively & ordered smaller seats for their 747s. It took years for airlines to recover, many just went bankrupt.

Mackk
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  • They were already flying as efficiently as they could, just to avoid having to land. Efficiently in terms of what? – bautista May 29 '19 at 06:03
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    @bautista - In terms of fuel consumption per mile flown. The idea is that if you fly trans-oceanic excessively fast, you will have to detour to land to refuel more often, and therefore truly long distance passengers will get to their destinations later while paying more. – Jirka Hanika May 29 '19 at 15:12
  • That's some anecdotal evidence you got there. I feel like an expert on the impact of the oil crisis on airlines just from reading that. Thank you. – Mad Physicist May 29 '19 at 19:47